Wide band interference absorber and technique for electromagnetic radiation

ABSTRACT

A multiple-stage interference-type absorber for extinguishing a wide band of high-frequency electromagnetic waves by interference. The absorber includes a reflecting base layer on which are superposed at least two further layers having in combination a total thickness of about lambda /2 wherein lambda is the average wavelength in the frequency band to be extinguished. One of the superposed layers is adjacent the base layer and consists of an embedding material in which fillers are embedded to impart to the material an increased relative magnetic permeability. This layer has a thickness which is less than lambda /4. The other layer is of a loss-free material.

United States Patent Meinke et al.

[54] WIDE BAND INTERFERENCE ABSORBER AND TECHNIQUE FOR ELECTROMAGNETIC RADIATION [72] Inventors: Hans H. Meinke, Arorstrasse 21, Munich; Kurt Ullrich, Elchingerweg 6, II, Ulm- Boflingen; Ludwig Weseh, Gaisbergstr 2, Heidelberg, all of Germany 221 Filed: April 11, 1967 21 Appl.No.: 634,020

52 1 us. Cl. ..343/18A [15] 3,680,107 [451 July 25, 1972 2,992,425 7/1961 Pratt ..343/1 8 A Primary Examiner-T. H. Tubbesing Attorney-Waters, Roditi, Schwartz & Nissen 57 ABSTRACT A multiple-stage interference-type absorber for extinguishing a wide band of high-frequency electromagnetic waves by interference. The absorber includes a reflecting base layer on which are superposed at least two further layers having in combination a total thickness of about M2 wherein A is the average wavelength in the frequency band to be extinguished. One of the superposed layers is adjacent the base layer and consists of an embedding material in which fillers are embedded to impart to the material an increased relative magnetic permeability. This layer has a thickness which is less than )t/4. The other layer is of a loss-free material.

8 Claims, 5 Drawing Figures l {58] Field of Search ..343/1 8 E, 18 A [56] References Cited UNITED STATES PATENTS 2,822,539 2/1958 McMillan "343/18 A PATENTEuJuLzs I972 3.680 107 sum 2 or 2 WIDE BAND INTERFERENCE ABSORBER AND TECHNIQUE FOR ELECTROMAGNETIC RADIATION DRAWING FIGS. 1 and 2 are impedance diagrams of known absorbers;

FIG. 3 is an impedance diagram of an absorber of the invention;

FIG. 4 is an impedance diagram of a modification; and

FIG. 5 illustrates the structure of an interference absorber according to the invention.

DETAILED DESCRIPTION This invention relates to wide-band interference absorbers.

Various types of absorbers are known which include one or more layers intended to attain adaption or matching to free space; i.e., they are intended to provide an iterative impedance of 377Q at their surfaces.

Adaptation absorbers are to be distinguished from interference type absorbers as provided by the invention. They will not provide cancellation of the waves by a difference in phase of waves double reflected by the absorber. Instead, waves are damped out by a gradual transition of the wave resistance within the absorber, without any occurrence of reflection at all. The conditions for dimensioning are therefore quite different in adaptation absorbers than in absorbers of the interference type and teachings known from the adaptation absorber field cannot be applied to absorbers of the interference type.

In addition, various kinds of )\/4 and nA/4interference absorbers are known with respect to which n must always be an uneven or odd multiple, since an effect opposite to that desired would otherwise occur. Further, it is a generally known fact that the band widths of such nA/4-interference absorbers are inversely proportional to the magnitude of the number n.

Transmission theory, however, permits designing a twocomponent absorber, constructed from a metal or metal-substitute layer, which in theory and in practice is superior to the above-described )t/4- and nA/4-absorbers; superior, that is, with respect to the width of the band on the one hand and with respect to the selection of the materials on the other. In the case of these nA/4-absorbers, the high-frequency or hf-constants of the materials as well as the thickness of the layer are critical and minute deviations inevitably produce mismatches.

It is an object of the invention to provide a k/Z-absorber which has a relatively simple structure, is not so critical with respect to the selection of the materials and to the thickness of the layers or coating and, in addition, offers the possibility of tolerable minor mismatches.

One of the differences between n/\/4- and A/2-absorbers is that the layers in the nk/4-absorber must be such that n fully complete )t/4-absorbers are superposed. In contradistinction thereto, the two components of the M2-absorber are not necessarily A/4-thick, but may be variable in their thickness, and it is merely necessary to fulfill the condition that the sum of the thickness of all layers is approximately A/2- for the complete matching operation. Further, it is possible to apply to a )JZ-absorber, additional layers 8,, B mB whose respective thickness amounts to about M4.

FIG. I shows, for purposes of comparison, a A/4-absorber in an argand diagram whereas FIG. 2 shows a 3lt/4-absorber. FIG. 3 illustrates the simplest case of a A/2-absorber, and FIG. 4, a )t/Z-absorber, to which two additional layers B, and B have been applied.

It should be noted that any interference absorber may be explained by an argand diagram or a Gauss plane which shows the real and imaginary portions of a \/4 absorber or even by a conventional Smith chart. Convention prescribes that when in a Gauss plane the real axis is reached by the curve there is a wave resistance in free space, i.e., 377 ohms as illustrated in FIGS. 1 to 4.

A substantial advantage of the A/Z-absorber resides in the fact that the wide band can be made still wider by applying additional components B to the A/Z-absorber. The layers B, 8,, B ....B, may vary as regards thickness and their high frequency properties.

The structure of such a A/Z-absorber is technologically simple and permits making allowance for existing materials, and can be computed for any wavelength desired.

Interference absorbers effect extinction in that roughly half of the waves are reflected at the upper surface of the absorber. The other half of the waves penetrating into the laminate are reflected at the reflecting base layer and interfere with the wave portion reflected at the upper surface of the laminate. For this purpose it is necessary that the distance between the outer surface of the laminate, where the first portion is reflected, and the reflecting base layer is M4, i.e., a quarter wavelength in the material, so that the portion of the waves penetrating into the laminate travels half a wavelength in passing through the laminate and back. Thereby the phases of the two interfering portions are exactly opposite to obtain wave cancellation.

The opposition of the phases of the two wave portions interfering at the outer surface is also obtained when the portion passing through the laminate and reflected at the base layer does not travel half a wavelength until it leaves the layer again, but 1%, 2%, 3%, etc., wavelengths, i.e., when the thickness of the laminate is n'A/4, n being any odd number. That means that nit/4 absorbers are usually interference absorbers which are thicker than normal M4 absorbers.

The present invention offers a completely different solution. In order to obtain a broader band, the invention uses an absorber combination, which is so constructed (see FIG. 5) that the thickness of the whole combination corresponds to the i value M4 for the longest wavelength range N of the frequency band to be influenced, whereas the thickness of the outermost layer of the layer combination corresponds to the value N4 of a wave A in the shortest range of the frequency band to be influenced.

Since the total thickness of the laminate roughly corresponds to a N74 of the range of the longest wavelengths, the thickness of the laminate is about M2 of a wave it of the medium frequency range. In particular A approximately equals A A /2. In other words, a known interference absorber layer tuned to the medium wavelength range A would, under the same conditions, be roughly half as thick as the laminate of the present invention for extinguishing this medium wave it. However, the laminate of the present invention is, moreover, also effective in a range of shorter waves, down to less than A" and up to a range of longer waves up to more than A so that satisfactory interference is obtained for an extraordinarily broad frequency band.

As will be discussed in greater detail hereinafter with reference to FIG. 5, the second part of the waves of the shorter N range is essentially reflected at the boundary layer between the outer layer and the inner layer of the layer combination. The longer waves A, however, must essentially pass beyond this boundary layer down to the reflecting base layer. Accordingly, the wave resistance increases by equal steps from free space at the outer surface of the laminate towards the reflecting base layer.

FIG. 5 shows the combination consisting of the reflecting base layer 1, the high-permeability magnetic layer A and the layer B which is free from losses. The incoming electromagnetic waves 7 reaching the interference type absorber of the invention will impinge first on layer B. In layer A, there are magnetic particles 2, which impart to this layer a permeability which is high compared to the permeability of free space, i.e., 1. The combination of layers comprising the layers A and B has a combined thickness of df M2, while the thickness of layer A is less than half the total thickness d, M4.

The component A is a layer with magnetic properties such as s, t, tanoe, tanSu, which are presumed to be known or can be measured. Said layer is composed of suitable plastic materials such as natural or synthetic rubber, synthetic resins,

varnishes and the like and is pigmented with large quantities of substances with high magnetic constants (powdered radiofrequeney or high-frequency iron, ferrites and so forth). The thickness of the layer is such that the resistance in the impedance plane becomes real.

The component B forms a loss-free (non-dissipative) layer and is characterized, therefore, by p. l and tan'o 0. This component must be so adjusted with respect to its 6 and (1,,- making allowance for the component A and the resulting R,. that complete matching adaptation is obtained.

Since the iterative impedance of the layer B (2 is proportional to l/ V2. can be calculated from the relation VR. 377 Z whereby RA is to be determined according to layer A If additional layers 8,, B WB (not shown) are applied to the k/2-absorber, their e and d must be adjusted with respect to each other and with respect to .the component A.

A method of calculating this, for example, the following:

B2=Ra3+ 31 315 The required thickness can be calculated, in a manner known per se, from the e of the partial layer.

When a metal-substituted layer is constructed, the component A can be replaced with a layer B, the e of which can be calculated according to the relation (where R is the value of the metal-substitute layer and R 3770) from the data of the metal-substituted layer.

A means the main-wavelength which has to be suppressed by the absorber.

e means the relative dielectric constant of the layer.

p. means the relative magnetic constant of the layer.

tane means the dielectric loss tangent of the layer.

tanou means the magnetic loss tangent of the layer.

d means the thickness of each layer.

Z means the wave impedance of each layer.

R is the real input impedance of the surface of each layer connected with the other layer or layers arranged on reflecting round. g Plastic and varnish material which are useful as embedding materials, are generally known for such purposes and are described by V. Hippel in Dielectric Materials, Technology Press of M.I.T., New York, 1954 and the following are useful in addition thereto:

Aniline formaldehyde Polyvinylalcohol acetate Polyethylmethacrylate Neoprene Cellulose acetate Polyethylene Polyisobutylene Polychlorotrifiuoroethylene Polytetrafluorethylene Polyisobutyl methacrylate Polyvinyl cyclohexane Natural rubber (Hevea rubber) Natural bitumen (Gilsonite) Polyethyl methacrylate.

As to the high-frequency iron referred to above, these are ferro-magnetic materials which have the desired constants in the desired wavelength range. A definition of the expression highfrequency iron" is as follows:

Iron reduced from carbonyls or other decomposable iron compounds with hydrogen, magnetite, yFe O ferrites, in which an iron is replaced with nickel, zinc manganese, etc.

Also useful is magnetite Fe O or iron oxide black,

produced by Badische Aniline- & Sodafabrik, Ludwigshafen. yFe- O by conversion of magnetite at 250 C. 1 atmosphere or also under pressure up to 1,000 atmos. can also be employed.

In order to select the desired hf-constant, it is possible to use in the embedding material, in addition, metal powders, semiconductor materials or the like, separately or in combination.

The metallic powders referred to above include: aluminum, beryllium, zinc, copper, manganese, cadmium, chromium, molybdenum, in particle sizes preferably of less than 100 1.1..

Semiconductor substances which can be used are: oxides of zinc, cadmium, magnesium, calcium; phosphides, antimonides and arsenides of indium, gallium; carbides, especially of calcium, silicon, titanium, aluminum, iron; silicides, carbonates (not soluble in plastics); silicates; sulfates (especially beryllium sulfate, calcium sulfate), phosphates; molybdates, tungstates; titanates; stannates; antimonates, arsenates, titanium dioxide, aluminum oxide, carbon black, graphite; chalcogenides of the second group of the periodic table, i.e., selenides, sulfides and tellurides of the element zinc, cadmium and mercury as defined in Concise Chemical and Technical Dictionary" edited by H. Bennett, New York, 1947.

The band widths comprise the range of 10 dB, i.e., where the decrease of the absorption amounts to only 10 dB. Maximal effect is indicated as main wave length, whereby an absorption of more than 30 dB is generally to be expected in same.

Examples of the structure of the invention are as follows:

EXAMPLE 1 Main wavelength 4 cm, band width 2.5 to 7 cm, construction on reflecting ground.

Component A: A butadiene-acrylonitrilcopolymer foil, pig mented with 76 percent by weight of magnetite of the particle size of less than 10 ,u of a layer thickness d, 2 mm;

Component B: PVC (polyvinyl-chloride) foil of the layer thickness D 5 mm.

EXAMPLE 2 Main wave length 10 cm, band width 6.25 to 17.5 cm,

erected on reflected ground.

Component A: Desmodur-/Desmophen-varnish 9 l, pigmented with percent by weight of hf-iron having the particle size of less than 5 p; thickness d, 4 mm;

Component B: Desmodur-lDesmophen-varnish 9 1, filled with 30 percent by weight oftalcum, d 12 mm.

Desmodur- 'Desrnophen varnish is a condensation product and is reacted as a two-component varnish after mixing and application, after dividing off of water, to form a clear, absolutely resistant and stable plastic (synthetic resin) varnish. Desmodur is an isocyanate, Desmophen acts as hardener and is produced in different qualities corresponding to the resulting kinds of varnish (different with respect to the mechanical hardness of the corresponding varnish). Desmophen is an adipic ester, mainly with phthalic acid and butane triol or butylene glycol, which produces the harder varnishes. The Desmodur-lDesmophen varnish is sprayed on or brushed on depending on the consistency. This requires a certain proportion of solvents which volatilizes fairly rapidly after applicatron.

EXAMPLE 3 Main wavelength 3.2 cm, band width 2 to 5.6 cm, construction of metal-substitute layer.

Metal-substitute layer: A polymerized isobutylene of BASF, Ludwigshafen, Germany, foil, filled with 30 percent by weight of carbon black 30 percent by weight of graphite, d 5 mm.

Component A: Chlorinated rubber foil, pigmented with 75 percent by weight of yFe O having a particle size from 10 to 20 1nd,, 8.2 mm;

Component B: Desmodur-lDesmophen varnish 9 1, filled with 20 percent titanium white, d 3.5 mm.

EXAMPLE 4 EXAMPLE 5 Main wavelength 5 cm, band width 3.15 to 8.75 cm, division of the component B into two partial layers (total thickness about 3 A4) construction on metal-substitute layer;

Metal-substitute layer: A butadiene-acrylonitrile-copolymer foil with 20 percent by weight of carbon black and 30 percent by weight of graphite, d 8 mm;

Component A: Polyethylene foil with 85 percent by weight of high-frequency iron, particle size 2 to 10 1.1., d, 2.7

Component Bl: Epoxy resin reinforced with glass fibers, d

Component B2: Silicone rubber foil d 8 mm.

EXAMPLE 6 Main wavelength 0.8 cm, band width 0.5 to 1.4 cm, erected on metal-substitute layers with two loss-free (non-dissipative) layers;

Metal-substitute layer: Polymerized isobutylene foil with 40 percent by weight of carbon black, d 2 mm;

Component A: Polymerized isobutylene foil d 1 mm,

Component B: Polytetra fluoroethylene foil, d 1.2 mm.

From the above, it will be seen that the subject matter of the invention is an absorber of the interference type. The absorber has a reflecting base layer and at least two further layers on top of the base layer. The said further layers have a total thickness of approximately dj' M2. The layer A which is adjacent the reflecting base layer has a thickness of d A/4. The layer A is provided with fillers which provide the layer with a relatively high magnetic permeability (p. l). The

other one of the two additional layers, i.e., layer B, consists of material free from losses.

What is claimed is:

l. A multiple-stage interference-type absorber for extinguishing by interference a wide band of high-frequency electromagnetic waves impinging on said absorber, comprising a reflecting base layer and at least two further layers on said base layer, said further layers on the base layer having a total thickness of about A/2, A being the average wavelength in the material of the frequency band to be extinguished, the one of said further layers adjacent said base layer including an embedding mass and a filler in said mass imparting to the material an increased relative magnetic permeability, said one layer having a thickness under A/4, the other layer being of a lossfree material.

2. An absorber layer according to claim 1, wherein said embedding means is a material having a relative dielectric constant of between 1 and 6.

3. An absorber according to claim 2, wherein said reflecting base layer is of a metal having a wave impedance of about zero for waves in said band.

4. An absorber as claimed in claim 3, wherein said embedding mass is at least one of the materials selected from the group consisting of aniline formaldehyde, polyvinylalcohol, acetate, polyethylmethacrylate, neoprene, cellulose acetate, polyethylene, polyisobutylene, polychlorotrifiuorethylene, polytetrafluorethylene, polyisobutyl methacrylate, polyvinylcyclohexane, natural rubber, natural bitumen and polyethyl methacrylate.

5. An absorber as claimed in claim 4, wherein the filler is a higih-frequenc iron.

. An absor er as claimed in claim 4, wherein the tiller IS a ferromagnetic material,

7. An absorber as claimed in claim 4, wherein the tiller is selected from the group consisting of aluminum, beryllium, zinc, copper, manganese, cadmium, chromium, molybdenum, in particle sizes of less than t.

8. An absorber as claimed in claim 4, wherein the filler is selected from the group consisting of oxides of zinc, cadmium, magnesium, calcium, phosphides, antimonides and arsenides of indium, gallium; carbides, especially of calcium, silicon, titanium, aluminum, iron; silicides, carbonates; silicates; sulfates, phosphates; molybdates, tungstates; titanates; stannates; antimonates, arsenates, titanium dioxide, aluminum oxide, carbon black, graphite; and chalcogenides of the second group of the periodic table.

t a a a a 

1. A multiple-stage interference-type absorber for extinguishing by interference a wide band of high-frequency electromagnetic waves impinging on said absorber, comprising a reflecting base layer and at least two further layers on said base layer, said further layers on the base layer having a total thickness of about lambda /2, lambda being the average wavelength in the material of the frequency band to be extinguished, the one of said further layers adjacent said base layer including an embedding mass and a filler in said mass imparting to the material an increased relative magnetic permeability, said one layer having a thickness under lambda /4, the other layer being of a loss-free material.
 2. An absorber layer according to claim 1, wherein said embedding means is a material having a relative dielectric constant of between 1 and
 6. 3. An absorber according to claim 2, wherein said reflecting base layer is of a metal having a wave impedance of about zero for waves in said band.
 4. An absorber as claimed in claim 3, wherein said embedding mass is at least one of the materials selected from the group consisting of aniline formaldehyde, polyvinylalcohol, acetate, polyethylmethacrylate, neoprene, cellulose acetate, polyethylene, polyisobutylene, polychlorotrifluorethylene, polytetrafluorethylene, polyisobutyl methacrylate, polyvinylcyclohexane, natural rubber, natural bitumen and polyethyl methacrylate.
 5. An absorber as claimed in claim 4, wherein the filler is a high-frequency iron.
 6. An absorber as claimed in claim 4, wherein the filler is a ferromagnetic material.
 7. An absorber as claimed in claim 4, wherein the filler is selected from the group consisting of aluminum, beryllium, zinc, copper, manganese, cadmium, chromium, molybdenum, in particle sizes of less than 100 Mu .
 8. An absorber as claimed in claim 4, wherein the filler is selected from the group consisting of oxides of zinc, cadmium, magnesium, calcium, phosphides, antimonides and arsenides of indium, gallium; carbides, especially of calcium, silicon, titanium, aluminum, iron; silicides, carbonates; silicates; sulfates, phosphates; molybdates, tungstates; titanates; stannates; antimonates, arsenates, titanium dioxide, aluminum oxide, carbon black, graphite; and chalcogenides of the second group of the periodic table. 